The present invention relates to a telecommunications system comprising at least one primary radio station and a plurality of secondary radio stations, in which a secondary radio station is matched to a primary radio station and a wireless communication between matched radio stations is done via a full duplex communication link, and the primary and the secondary stations are arranged for establishing the communication link to their respective opposite station while scanning for a free communication channel, whereby the stations have a transmitter and a receiver. Such a telecommunications system can be a cordless telecommunications system in which radio stations are matched to each other via an identification code, or any other suitable system in which matched radio stations communicate with each other.
The present invention further relates to a channel selection protocol and a radio station for use in such a system.
A telecommunications system of the above kind is a known CT0 or CT1 cordless telephone readily available on the market. The known system can be a CT1 cordless telephone suitable for the European market. Such a system is standardised by ETSI, and the technical characteristics can be found in the ETSI document I-ETS 300 235, pages 9-16, March 1994. In such a system a full duplex radio channel is defined as a pair of channels with the portable part and for the fixed part. When a radio station is scanning for a free channel it checks whether the measured field strength is below a given threshold. The known system can also be a cordless telephone suitable for the US market For such a system the FCC makes available frequencies. In the latter system the radio channels for the fixed and portable part are not paired. In both systems there is a channel selection protocol to prevent call establishment of an occupied channel. In a system of paired channels, when a station scans for free channels when establishing a full duplex communication link, a corresponding channel is automatically taken. In the system of unpaired channels, the FCC prescribes that no occupied channel may be seized but no details are given how to achieve this object. Particularly in a system of unpaired channels, a reliable protocol is needed for up a fill, duplex communication link. Also in a system of paired channels, a problem can arise when automatically seizing a corresponding channel in the pair of channels. Specifically a busy channel adjacent to a free channel of the corresponding channel in the channel pair may with the free channel. In such a case half of the duplex link can be of such a bad quality that the communication is seriously disturbed.